Typically, pile drivers and hydraulic hammers incorporate a weight which is carried upon a guide frame for reciprocating travel. The weight is raised against gravity by an hydraulic ram to which high pressure fluid is applied to extend the ram. When the weight has been raised to the desired extent, the high pressure fluid is vented from the ram and the weight is allowed to fall under gravity upon the pile, ground compaction foot, ground breaker tool or other object upon which the weight is to act. The hydraulic ram can act directly upon the weight, for example as when the weight is attached to the piston rod of the ram and is raised as the piston within the cylinder of the hydraulic ram is raised. Alternatively, the hydraulic ram can act indirectly upon the weight, as when the weight is attached to the piston rod of the hydraulic cylinder by a rope which passes over a pulley at or adjacent the top of the guide frame or as when the hydraulic cylinder acts upon the end of a lever arm connected to the weight.
The operation of the hydraulic ram serves to raise the weight against gravity to the desired extent to achieve the desired impact blow upon the object being acted upon when the ram is allowed to contract. The object can be, for example the top of a pile which is to be driven into the ground, a ground compaction foot which is used to compact or level the ground, or an earth or concrete breaker tool which it is desired to subject to a linearly acting impact blow.
For convenience the term hydraulic hammer will be used herein to denote apparatus of the above type in general in which an object is subjected to a linearly acting impulse blow by a weight which is reciprocated by means of an hydraulic ram.
The size of the impact blow will depend upon the mass of the weight and the velocity of the weight at the moment of impact with the object being struck. With a weight which is raised against gravity by a single acting hydraulic ram and falls under gravity, the velocity will depend upon the height to which the weight is raised. Practical considerations may limit the mass which can be raised by a given hydraulic ram and the height to which the apparatus can extend.
It has therefore been proposed to use a double acting ram in which the weight is raised by one part of the cycle of operation of the ram (the rising stroke of the ram) and then positively driven in the opposite direction by a second part of the cycle of the ram operation (the falling stroke of the ram). Whilst such double acting rams may achieve a greater impact blow due to the positive drive imparted to the weight by the ram during the falling stroke of the ram, the need to regulate the flow of hydraulic fluid to and from the ram introduces complexity in the fluid control system and requires the use of high and low pressure accumulators to enable the high flow rates of high and low pressure to and from the ram cylinder to ensure an adequate rate of motion of the weight on its upward and downward travel and to enable a rapid rate of repetition of the impact blows to be achieved.
The energy available at impact of the weight upon the object is dependent upon the velocity which the weight attains at impact. Typically, in a hammer as used in a rock breaker or drill, the weight which is being driven by the double acting ram is comparatively light, often no more than the weight of the object against which it is being driven. In order for such a light weight to acquire a high energy in a short distance of travel, the weight must be subjected to high acceleration by the ram. This also results in a short time for the ram to complete its stroke. As a result, particularly in such applications of an hydraulic ram, it is necessary to ensure that fluid is fed to the ram at high pressures to achieve the necessary acceleration and that the rate of flow of fluid to and from the ram is high to allow the ram piston to move rapidly within the ram cylinder. This requires the use of large and powerful fluid pumping systems and the use of high and low pressure accumulators to achieve the desired flow rates of high and low pressure fluids to and from the cylinder of the ram. These components have added to the weight, size and complexity of the hammer assembly, over and above the hydraulic ram and the weight. Where the hammer is to be transportable, it is necessary to provide support machines, for example cranes or tractors to support and carry the hammer mechanism over the ground at sites where the hammer is used, for example to achieve some form of work on or in the ground, for example soil compaction, pile driving, rock drilling or concrete slab break up. The need for large support machines adds to the cost and complexity of the equipment.
In place of a double acting ram, it has been proposed to lift the weight against a coil compression spring using a single acting hydraulic ram, so that the spring provides a positive downward force when the lifting of the weight by the ram has been completed and the weight is released for downward travel. Such a spring has to be large and heavy to provide the necessary downward force to be practicable and provides little benefit over the use of a conventional single acting ram powered mechanism which achieves the same impact blow with the same weight.
We have now devised a mechanism by which a single acting hammer mechanism can readily be provided with additional energy storage means to provide the driving force on the falling stroke of the hydraulic ram and thus enhance the velocity of the weight upon impact with the object which it is to strike. The invention thus provides an alternative to the use of a double acting hydraulic ram, notably in applications such as rock or concrete drills or breakers, using a single acting hydraulic ram. The invention reduces the need for and/or the size and weight of any high and/or low pressure hydraulic accumulators which may be required as compared to a double acting ram and enables a lighter and simpler overall hammer mechanism to be achieved, thus reducing the required size and weight of the support machine whilst achieving an impact blow significantly greater than that achieved using a single acting ram raising the weight to the same height.
The invention can also be applied to testing of rigid structures in which the structure is deflected by an applied deflection force from its rest position against a biassing load force, is released at a predetermined degree of deflection, and is allowed to flex repeatedly under the biassing force and the opposing forces due to the rigidity of the structure and/or due to the cyclically applied opposed deflection forces. For example, the invention can be applied to the fatigue testing of elongated structures, such as aircraft wings, which are subjected to cyclically varying deflection forces whilst being subjected to a continuous biassing force.